The failure to resolve infectious diseases as well as to fight autoimmune diseases often results from inappropriate, rather than from insufficient, immune responses. Thus, it is critical for vaccine design to be able to manipulate the type of response induced. We are proposing a comprehensive analysis of parameters that contribute to determining the direction of the T cell cytokine secretion profile. We will explore how dominance/crypticity, and availability/affinity affect determinant display, as well as ways in which the mode of antigenic presentation--in what strain, with what cytokines, by what route--combine to influence Th1/Th2 choice. What is the relationship of immunodominance to Th1/Th2 choice and induction or protection from disease? Based on our previous evidence that MHC-binding affinity of the determinant can impact the T cell cytokine secretion profile, we will focus on the role of antigen itself. We will modulate the MHC-binding affinity of determinants of the LACK antigen or Leishmania major and of myelin basic protein (MBP), in order to induce a Th1 or Th2 response to protect mice from L. major infection or MBP-induced experimental allergic encephalomyelitis (EAE), respectively. The relative importance of MHC-binding affinity, adjuvant, cytokine milieu (including adenovirus-mediated cytokine gene delivery) and mouse genetic background will be assessed. We will also address the impact of such manipulations on the T cell repertoire. From Vbeta single chain transgenic T cells, specific for peptide (161-173) of LACK, T cells with different Valpha genes and of widely disparate affinities will be selected and 3 new transgenic chains will be prepared expressing these TCRs. We will determine whether both high avidity and low avidity T cells have the capacity to affect Th1/Th2 choice in the presence of high and low MHC- affinity ligands. Whether high and low affinity, MHC ligands address distinct populations of T cell swill be determined using immunoscope analysis, which allows visualization of the T cell response based on distinct CDR3 lengths. The role of T cells specific for a subdominant determinant within the responses based on distinct CDR3 lengths. The role of T cells specific for a subdominant determinant within the MBP121-150 region, recruited early during determinant spreading, will be studied. This region consists of two overlapping determinants that possess differential MHC-binding affinities. We will analyze whether these determinants induce T cells of overlapping common specificity versus T cells of private or flanking specificity. This study will allow us to define the relative roles of MHC affinity and TCR avidity in steering the response in a protective versus an aggressive direction.